1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire for truck and bus, and more particularly to an improvement of the durability in the bead portion of the heavy duty pneumatic radial tire continuously running at a high speed under a heavy load over a long distance.
2. Related Art Statement
In general, when the heavy duty pneumatic radial tire mounted onto a truck, bus or the like is continuously run at a high speed under a heavy load over a long distance, bead failure occurs. This bead failure produces a crack between the turnup end portion of the carcass ply and the surrounding rubber, which then grows to cause separation failure.
When steel cords are used in the carcass ply of the heavy duty pneumatic radial tire, cracking at the turnup end portion of the carcass ply is caused by the stress concentration at the turnup end portion due to the deformation of the bead portion during the running under a heavy load, and the decrease of adhesion force between steel cord and rubber due to the rise of rubber temperature at the turnup end portion.
Heretofore, a bead portion of the tire as shown in FIG. 1 has been proposed in order to simultaneously establish the suppression of the deformation of the bead portion and the reduction of heat build-up at the turnup end portion of the carcass ply. In FIG. 1, numeral 1 is a heavy duty pneumatic radial tire for use in truck, bus or the like. The tire 1 comprises a bead portion 3 having each of bead cores 2, and a carcass 5 composed of a main carcass body 5a containing rubberized steel wire cords, which are extended between the bead cores 2 substantially in a radial plane of the tire and wound around the bead core 2 from the inside to the outside to form a turnup portion 5b. Between the main carcass body 5a and the turnup portion 5b of the carcass 5 are arranged a rubber stock A.sub.0, 6 with a triangular section having a high elasticity of modulus and contacting with the bead core 2 for suppressing the falling-down deformation S of the bead portion 3 under a heavy load, and a rubber stock B.sub.0 , 7 having a low internal loss and extending outward in the radial direction outside the axial direction of the rubber stock A.sub.0 for suppressing the heat build-up of the bead portion 3. The rubber stocks A.sub.0 and B.sub.0 form a stiffener 8. Numeral 9 is a chafer, and numeral 10 is a sidewall.
As mentioned above, in the conventional technique, the falling-down deformation of the bead portion 3 is suppressed and the heat build-up in the vicinity of the turnup end portion 5c is reduced by dividing the stiffener 8 into two layers.
In the recent heavy duty pneumatic radial tires, however, the tire life is largely increased with the improvement of the wear resistance and the increase of the retreading number, and consequently the number of repetitive deformation at the bead portion 3 under a heavy loading largely increases. Further, since the change of structure and shape in the tire is made for reducing the weight of the tire and the production cost, the rigidity of the bead portion lowers to make the falling-down deformation S of the bead portion large, and the strain in the vicinity of the turnup end portion 5c of the carcass ply 5 increases. As these results, a small crack is produced at the turnup end portion 5c and gradually grows inward in the bead portion to finally produce a large crack failure 4 as shown in FIG. 1.
In this connection, Japanese Patent Application Publication No. 54-564 discloses that the turnup portion of the carcass ply is bent so as to be made long with respect to the minimum distance between the turnup end and the bead core, whereby the separation failure between the turnup end and the rubber is prevented and the bead durability is improved. In this article, there is discussed the arrangement of the carcass turnup portion when particularly using a light carcass made from an organic fiber such as polyamide or the like as a carcass ply cord.
On the other hand, when the turnup end of the carcass ply is separated away from the carcass ply itself as far as possible, the amount of rubber stock interposed between the carcass ply and its turnup portion can be made large to reduce the stress concentration in the turnup end. In this case, however, when steel wire having a high rigidity is used as a ply cord, there is caused a problem inherent to the steel.
In general, when the tire is inflated under an internal pressure, the bead portion is elongated as a whole, during which the difference of the rigidity is produced between the upper part and lower part of the bead portion at the boundary of the rigid turnup portion, and consequently the elongating of the surface rubber layer in the bead portion becomes large above the turnup end portion rather than beneath thereof and the surface rubber layer is apt to be dented. As the tire runs on road under a loading, the turnup portion of the carcass ply is gradually put inward in the radial direction of the tire and also the surface rubber layer near to the turnup end is followed thereto, so that the fatigue failure is finally produced in the above dent through repetitive deformation. As a result, not only cracks are produced from the dent, but also the separation failure is caused in the end portion of the turnup due to the elongating. Thus, the durability of bead portion in this type of the tire is degraded by the synergistic action of these failures.